Must Have Firefox Extensions

[oz]

Hey guys, we've all posted about various Firefox extensions in the past, but new ones are coming out all the time, while others die off or lose popularity.

My own must have extensions would include:

Adblock Plus (easy blocking of unwanted ads)

Element Hiding Helper (blocks any unwanted webpage elements)

United States English Dictionary (spellchecker for Firefox)

All the above have spoiled me and I really have a rough time browsing the web without them.

What are your current "must have" extensions for Firefox, and what do they do?

[L4Linux]

Video Download helper
This extension allows you to download flash videos from Youtube, as well as dozens of sites. 264.000 downloads per week speak for it. It updates very often to support more sites.
Download statusbar
If the pop-up window that shows you the downloads annoys you, then download statusbar is for you. It replaces that window with a bar at the bottom of Firefox, which is not obtrusive.
DownThemAll!
A simple to use download manager inside Firefox. It supports resume as well as downloading from many sources simultaneously. Personally, i don't need much more from a download manager.
Fission
This is more eycandy, than usability. It adds a progress meter to the web bar.
Foxmarks
Foxmark allows you to synchronize your bookmarks across many PCs.
Google Preview
Google preview can help increase your productivity by showing a Thumbnail beside each Google result. Want even more? It works with Yahoo as well!!
Tab Scope
It shows a preview of tabs, as you hover your mouse over them.

Themes
I use Strata Human,
which gives the key-lock shape that we all like in Windows and Mac. For some reason beyond comprehension, the Linux version does not come with this shape in the back/forward buttons and looks like Firefox 2 at first sight.

For KDE users, i suggest KDE4 + Firefox 3 theme. It gives Firefox 3 an Oxygen look.
2 other nice themes are MacOS X for those who like Mac visual style and Phoenity Reborn.

[daacosta]

Some of the extensions I use that have not been mentioned are:

Zotero
Bookmark this page plus
Openbook

[GNU-Fan]

There is only one must have for me and this is

NoScript (blocks execution of JavaScript, Flash and Java).

I simply cannot stand the "Web 2.0" without it.

[Freston]

Adblock Plus Obviously
NoScript Dito

Download Statusbar I like this one.
Locationbar I've gotten very used to this one, and only notice it when it's not there. It highlights the domain in an URL, for easy viewing.
External IP Good to know your own IP.
Temporary Inbox My main defense against spam.
Server Spy This one is a lot of fun, it shows what webserver is serving the site you're viewing. In case of LFo: Apache/1.3.41

And of course a couple of dictionaries.

[elija]

Those that I haven't seen mentioned

Firebug - Debugging and inline editing of HTML / CSS etc
WebDeveloper Toolbar - Does exactly what the name imples

[aluminumspleen]

NoScript and AllInOne Sidebar are my must-haves.

A cool one that I havent had a use for yet but still seems neat is User Agent Switcher, that makes servers think you are running IE, Netscape, or whatever you choose.

[schwim]

abp
element hiding helper
filterset.g updater
tab mix plus
fireftp
noscript
refspoof
user agent switcher
web developer
unhide passwords

I carry a Firefox install on a USB stick so I don't have to use others' browsers without them.

[jayd512]

No-Script and Ad-Block are the main ones. I used User Agent Switch for a while at work.
Other than that I was using Tab Effects while still on FF2. Just a little useless eye-candy there that I hope gets to find a place in FF3.

I forgot to mention ChatZilla. I don't show up in IRC very often, but it's nice to have!

[darkrose0510]

firebug
developer toolbar
javascript debugger
live http headers
dom inspector